Warm Air Thru Center Vents When Heater's Off in 380sl

My 81 380sl is pushing warm air out of the center vents, even when the heater control is on "OFF." However, if I turn the A/C button on and
push in either the HI or Low switch, the air will get cold. But I don't think its getting as cold as it should, probably because its mixing with the warmer air. Otherwise, the system seems to be working fine. Anybody know where to check first for a vacuum leak?
Thanks vary much in advance.
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